Women are suffering escalating levels of illegal discrimination at work when they get pregnant, and are often made redundant while they are on maternity leave, according to a new poll.

The figures show one in seven of the women surveyed had lost their job while on maternity leave; 40% said their jobs had changed by the time they returned, with half reporting a cut in hours or demotion. More than a tenth had been replaced in their jobs by the person who had covered their maternity leave.

Samantha Mangwana, an employment lawyer at Slater & Gordon, the law firm which commissioned the research, said the results were "sad and shocking".

"Women are suffering in silence," she said. "A common case is that a woman goes back to her role and all her clients have been given to other people. And they are not returned. So everything she has built up over the years is gone. Or they are simply being made redundant ahead of worse-performing men. The big issue is that women are somehow seen as being less committed to their employers because they are now mothers. Many companies are settling out of court because they don't want to be seen to be treating pregnant women or new mothers like this. But the awful thing is that I see the same major companies again and again and again, writing out these cheques – accompanied, of course, with a confidentiality clause."

Research company OnePoll questioned 1,000 women last month. On returning to their jobs, almost a third of the new mothers (30%) felt they didn't fit in any more and two in five felt they lacked support, with almost 20% feeling that no one understood what it was like juggling work with new motherhood. Nearly one in 10 said the stress affected their relationship with their partner. Only 3% had sought legal advice over maternity discrimination; 10% had sought help from their HR department.

It is unlawful to dismiss or otherwise disadvantage an employee for a reason related to her pregnancy or maternity leave. Campaign group Maternity Action provides advice to women and is trying to get the government to monitor unlawful discrimination. Before the recession, the Equal Opportunities Commission estimated that 30,000 women lost their jobs each year as a result of being pregnant, and campaigners believe that figure has risen dramatically.

"Those walking into Slater & Gordon are the tip of the iceberg," said Rosalind Bragg, director of Maternity Action. "We know there's a huge amount of pregnancy discrimination among low earners, who would not be able to go into a legal office for help. Demand for our helpline has doubled year on year for the past three years and our information sheets were downloaded 397,000 times.

Few cases go all the way to tribunal and when they do they attract a lot of publicity, like the case last month of Katie Tantum, 33, a trainee solicitor who accused a City law firm of sex discrimination, saying they "just stopped bothering" with her when they discovered she was pregnant. A judgment has yet to be reached with the firm claiming it was her "intellectual vigour" at fault.

Bragg said: "We believe only a very small percentage of women take any action against an employer who has broken the law. They may simply not know their rights, but often there will have been a period of bullying and harassment before the woman is finally sacked or leaves, so she's in no position to invest energy and emotion, let alone money, in pursuing an action that may well not produce results."

She pointed to comments made in 2011 by Downing Street's then director of strategy, Steve Hilton, who suggested that scrapping all maternity allowances at work would help the economy. "People might have backed away from his comments afterwards, but that message was already out there loud and clear." She added: "This year employment tribunal fees are to be introduced: an upfront fee of £1,200 before you even start, which is just increasing barriers. We expect the situation to get a lot worse."

One woman who contacted Maternity Action was Sarah, from the south-east. She wrote: "I was both anxious and excited about going back to work … I'd agonised over childcare and paid a large deposit at our local nursery. I'd left my son there for two weeks to get him used to it gradually. I had to buy new clothes as none of my old ones fitted any more … Like many women, I felt unconfident about going back – but also exhilarated. The day came, I dropped my baby off, and arrived at work early and raring to go. The head of personnel invited me for a coffee to explain that, with regret, I was being made redundant."